Molecular modification of coumarin dyes for more efficient dye sensitized solar cells

J. Chem. Phys. 136, 194702 (2012); h ps://doi.o g/10.1063/1.4711049 136, 194702 © 2012 Ame ican Ins i u e o Physics. Molecula modi ica ion o couma in dyes o mo e e icien dye sensi ized sola cells Ci e as: J. Chem. Phys. 136, 194702 (2012); h ps://doi.o g/10.1063/1.4711049 Submi ed: 01 Feb ua y 2012 . Accep ed: 08 Ap il 2012 . Published Online: 16 May 2012 Rocío Sánchez-de-A mas, Miguel A. San-Miguel, Jaime O iedo, and Ja ie Fdez. Sanz ARTICLES YOU MAY BE INTERESTED IN Couma in dyes o dye-sensi ized sola cells: A long- ange-co ec ed densi y unc ional s udy The Jou nal o Chemical Physics 129, 214703 (2008); h ps://doi.o g/10.1063/1.3025924 Densi y- unc ional he mochemis y. III. The ole o exac exchange The Jou nal o Chemical Physics 98, 5648 (1993); h ps://doi.o g/10.1063/1.464913 Time-dependen densi y unc ional heo y in es iga ion o he abso p ion, luo escence, and phospho escence spec a o sol a ed couma ins The Jou nal o Chemical Physics 125, 164324 (2006); h ps://doi.o g/10.1063/1.2361290 THE JOURNAL OF CHEMICAL PHYSICS 136, 194702 (2012) Molecula modi ica ion o couma in dyes o mo e e icien dye sensi ized sola cells Rocío Sánchez-de-A mas,a) Miguel A. San-Miguel, Jaime O iedo, and Ja ie Fdez. Sanz Depa men o Physical Chemis y, Uni e si y o Se ille, Se ille, Spain (Recei ed 1 Feb ua y 2012; accep ed 8 Ap il 2012; published online 16 May 2012) In his wo k, new couma in based dyes o dye sensi ized sola cells (DSSC) ha e been designed by in oducing se e al subs i uen g oups in di e en posi ions o he NKX-2311 s uc u e. Two ypes o subs i u ions ha e been conside ed: he in oduc ion o h ee elec on-dona ing g oups (–OH, –NH2, and –OCH3) and wo di e en subs i uen s wi h s e ic e ec : –CH2–CH2–CH2– and –CH2–HC=CH–. The elec onic abso p ion spec a (posi ion and wid h o he i s band and ab- so p ion h eshold) and he posi ion o he LUMO le el ela ed o he conduc ion band ha e been used as heo e ical c i e ia o e alua e he e iciency o he new dyes. The in oduc ion o a –NH2 g oup p oduces a edshi o he abso p ion maximum posi ion and he abso p ion h eshold, which could imp o e he cell e iciency. In con as , he in oduc ion o –CH2–CH2–CH2– does no modi y signi ican ly he elec onic s uc u e o NKX-2311, bu i migh p e en agg ega ion. Finally, –CH2– HC=CH– p oduces impo an changes bo h in he elec onic spec um and in he elec onic s uc u e o he dye, and i would be expec ed as an imp o emen o cell e iciency o hese dyes. © 2012 Ame ican Ins i u e o Physics.[h p://dx.doi.o g/10.1063/1.4711049] I. INTRODUCTION Couma in dyes a e one o he mos p omising classes o o ganic sensi ize 1–4 o hei use in dye sensi ized so- la cells5–7and hey ha e been sys ema ically s udied by Ha a and A akawa.8–14 They a e based on he concep o he dono -accep o π-conjuga ed s uc u e (D-π-A), wi h he couma in skele on con aining an amino g oup as he dono and a cyanoac ylic acid as he accep o componen . They a e conside ed ype-I dyes, in which elec on injec ion occu s h ough an indi ec mechanism: he e is a pho oexci a ion o he dye o an exci ed s a e, ollowed by he elec on injec ion om his s a e o he semiconduc o conduc ion band. A d awback ela ed o hese couma in dyes is ha hey a e liable o expe ience π-s acked agg ega ion on TiO2su - aces. This ype o agg ega ion would be ad an ageous o ligh ha es ing, bu i usually leads o ine icien elec on in- jec ion because o in e molecula ene gy ans e p ocesses be ween he dyes, esul ing so in low cell e iciency. A s a - egy o p e en dye agg ega ion is adding coadso ba es, like deoxycholic acid and 4- e -bu ylpy idine.13 Al hough hese addi i es imp o e he elec on injec ion, hey signi ican ly de- c ease dye adso p ion limi ing he cell pe o mance. Ano he s a egy ha has been ecen ly es ed is o in oduce a side ing in he molecule, linked o he alkene chain. This educes he dye agg ega ion while keeping he adso bed dye amoun .14 Al hough couma in based dyes ha e been ex ensi ely s udied in ecen yea s, he e a e no many heo e ical s udies abou hem.15–25 I is wo h no ing ha up o da e, all p e i- ous heo e ical wo ks a e limi ed o he s udy o he elec onic s uc u e and he op ical p ope ies o ee dyes and none o hem ackles he in luence o he adso p ion p ocess on he a)Au ho o whom co espondence should be add essed. Elec onic mail: [email p o ec ed]. elec onic abso p ion spec a o hese dyes. Howe e , in se - e al ecen wo ks, i has been demons a ed ha he dye ad- so p ion p ocess on he semiconduc o p oduces a eo ganiza- ion o he elec onic s a es and his ac in oduces impo an changes in he elec onic abso p ion spec a.26–28 The e o e, o heo e ically de e mine he con enience o using hese dyes as DSSC sensi ize s, i is necessa y o include he semicon- duc o e ec explici ly in he model. In addi ion, he clus e size should be la ge enough o ep oduce well he elec onic s uc u e o adso bed dyes. In a e y ecen wo k, se e al couma in dyes ha e been heo e ically s udied wi h he aim o inding a ela ionship be- ween some heo e ically calcula ed p ope ies and he e i- ciency o hese dyes as DSSC sensi ize s.29 F om hese e- sul s, some pa ame e s such as he posi ion and wid h o he i s band in he elec onic abso p ion spec a, he abso p ion h eshold and he LUMO ene gy espec o he conduc ion band edge ha e been chosen o es ablish use ul c i e ia o e alua e he e iciency o hese molecules as sensi ize s in DSSCs. These c i e ia allowed e alua ing, in good ag eemen wi h expe imen al e idences, he e iciency o de o he s ud- ied molecules. In his wo k, an example o how he p e iously es ab- lished c i e ia can be employed o p edic he e iciency o new dyes is shown. Ou aim is o design new, mo e e icien sensi ize s, educing economical cos and syn hesis e o . We ha e conside ed he NKX-2311 dye, which p esen s a good e iciency as sensi ize (5.2%) and whose elec onic abso p ion spec um has been p e iously s udied in de ail and has been ep oduced accu a ely.29 The calcula ed abso p ion spec um o he ee NKX-2311 p esen s a main in ense band a 2.30 eV wi h a small shoulde a highe ene gies. This alue is in good ag eemen wi h he expe imen al spec a, which p esen s he main band a 2.42 eV when he spec um 0021-9606/2012/136(19)/194702/7/$30.00 © 2012 Ame ican Ins i u e o Physics136, 194702-1 194702-2 Sánchez-de-A mas e al. J. Chem. Phys. 136, 194702 (2012) is egis e ed in me hanol solu ion.8,9The main e ec ha can be obse ed in he spec a a e adso p ion is a widening o he i s band, while he maximum posi ion emains almos unal e ed. The main band o he adso bed NKX-2311 spec um appea s cen e ed a 2.23 eV and is con ibu ed om se e al exci a ions om HOMO o bi al o di e en o bi als delocalized o e he clus e . Ne e heless, all exci a ions wi h conside able in ensi y in ol e he LUMO+5 o bi al, which is comple ely localized on he molecule, e y simila o he LUMO o he ee dye. Wi h he aim o designing dyes wi h highe e iciency han NKX-2311, se e al unc ional g oups ha e been in o- duced in di e en posi ions o his molecule and i s e ec on he elec onic s uc u e and he abso p ion spec a o he dyes ha e been analyzed. II. COMPUTATIONAL METHODS AND MODELS Two complemen a y se s o calcula ions ha e been ca - ied ou o each model: (1) DFT geome y op imiza ions ha e been done by us- ing he Pe dew–Bu ke–E nze ho (PBE) unc ional30 oge he wi h no m-conse ing pseudopo en ials31 in he ully non-local Kleinman-Bylande 32 o m and an auxilia y eal-space g id equi alen o a plane-wa e cu -o o 130 Ry. A non-s anda d DZP (double ze a plus pola iza ion o bi als) basis se o Na u al A omic O bi als (NAOs) cons uc ed om he eigens a es o he a omic pseudopo en ial was used.33–35 The op ical esponse was hen compu ed using eal ime TD-DFT ( ime domain) simula ions wi hin he SIESTA (Spanish Ini ia i e o Elec onic Simula ions wi h Thousands o A oms) implemen a ion.26,36–38 We conside , as an ini ial s a e, a sys em in a ini e elec ic ield o 0.1 V/Å. Each sys em was allowed o e ol e du ing 36 s wi h a ime-s ep o 1.5 ×10−3 s. The damping ac o used in he Fou ie ans o ma ion was 0.1 eV. (2) Addi ionally, linea esponse (LR) equency domain calcula ions we e pe o med in he gas phase using he PBE unc ional and he 6-31G (d,p) basis se . The op ical spec a we e simula ed by con en ional TD-DFT ( e- quency domain) using GAUSSIAN 03.39 O e 400 single ansi ions we e needed in selec ed calcula ions. Be o e he spec um calcula ion, ib a ional equency calcula- ions we e done o con i m he s abili y o all he op i- mized geome ies. The e a e some ad an ages associa ed wi h he eal- ime TD-DFT o mula ion: all he possible exci a ions a e gene - a ed a he same ime so he spec um is ob ained in a wide ange o ene gy; he implemen a ion is ela i ely simple and al hough he compu a ional cos is no cheap o small sys- ems, i becomes compe i i e wi h adi ional me hods as he sys em size inc eases. Howe e , he main disad an age is ha i is no possible o cha ac e ize he na u e o he di e en ansi ions because he op ical esponse is ob ained om he analysis o he elec onic dipole momen , whe e he in o - ma ion om he wa e unc ions is in eg a ed. Fo his ea- son, he combina ion wi h equency domain TD-DFT is e y in e es ing. In a p e ious wo k, he e ec o he exchange-co ela ion unc ional and he sol en has been es ed o couma in dyes, and pa icula ly o he NKX-2311.29 Conclusions o ha wo k ha e been ex apola ed o he dyes conside ed he e, which ha e simila s uc u es. The equency domain TD- DFT spec a ha e been calcula ed o he ee NKX-2311 us- ing he PBE and B3LYP unc ionals and wi h and wi hou sol en e ec s (PCM model).40,41 This model p o ides eli- able abso p ion ene gies as long as no speci ic in e ac ions link he solu e and he sol en molecules.42,43 The posi ion o he main peak is shi ed o highe ene gies when passing om he PBE o he B3LYP unc ional, in ag eemen wi h p e i- ous published da a.44 Compa ed o expe imen al spec a, PBE p o ides be e esul s han B3LYP o NKX-2311 (al hough B3LYP wo ks be e when he molecule size inc eases). The inclusion o me hanol sol a ion e ec s h ough a PCM model ends o educe he exci a ion ene gies (abou 0.2 eV), due o a s abiliza ion o he LUMO o bi al. Compa ing wi h he p e- ious wo k, we ha e also checked ha he inclusion o long- ange co ec ions in he unc ional and he use o addi ional di use base unc ions do no imp o e subs an ially he esul s o his pa icula dye. This is a consequence o he kind o ex- ci a ion we a e dealing wi h. As men ioned ea lie , his kind o molecules wo k on an indi ec mechanism (in amolecula exci a ion ollowed by an elec on injec ion o he conduc- ion band o he semiconduc o ). The molecules conside ed in his pape a e no la ge enough o p esen a long- ange in- amolecula cha ge ans e (which could be unde es ima ed by TD-DFT) and he e o e, o hese molecules, he pho oex- ci ed s a e shows a mode a e cha ge ans e . Since o his conc e e sys em he e is no a clea choice, we ha e pe o med calcula ions wi h he PBE unc ional, which allows o an eas- ie compa ison be ween eal- ime and equency calcula ions. An ex ended desc ip ion o he sol en and unc ional in lu- ence is shown in Re . 29. Se e al subs i uen g oups ha e been in oduced in o di - e en si es o he NKX-2311 s uc u e wi h he aim o im- p o ing i s e iciency as sensi ize . The subs i u ion si es a e shown in Figu e 1. Two ypes o subs i u ions ha e been con- side ed. Fi s , h ee di e en elec on-dona ing subs i uen s ha e been in oduced in d posi ion, –OH, –NH2, and –OCH3 and he new dyes ha e been labelled as D1, D2, and D3, e- spec i ely. In a p e ious heo e ical wo k, i is sugges ed ha his kind o subs i u ion in ha posi ion could lead o an im- p o emen o cell e iciency, due o a des abiliza ion o he HOMO o bi al.21 Second, we ha e in oduced in he NKX- 2311 molecule wo di e en subs i uen s wi h s e ic e ec : ON O COOH CN d abce FIG. 1. Subs i u ion si es on NKX-2311. 194702-3 Sánchez-de-A mas e al. J. Chem. Phys. 136, 194702 (2012) TABLE I. Se e al subs i uen s ha e been in oduced in di e en posi ions o he NKX-2311 molecule o design new couma in based dyes. Dye Subs i uen and posi ion D1 d =–OH D2 d =–NH2 D3 d =–OCH3 AB1 a,b = BC1 b,c = CE1 c,e = AB2 a,b = BC2 b,c = CE2 c,e = –CH2–CH2–CH2– (1) and –CH2–HC =CH– (2). Two g oups o each subs i uen ha e been in oduced in he NKX-2311 in h ee di e en combina ions o posi ions (a and b, b and c, and c and e) esul ing in six di e en dyes: AB1, BC1, CE1, AB2, BC2, and CE2. These models a e summa ized in Table I. The compu a ional me hodology has been applied o he ee dyes and he same dyes adso bed o a (TiO2)9clus e . The s a ing geome y o his clus e was aken om he li e a u e.45,46 I was ob ained ia he geome y op imiza- ion o he o iginally sphe ical shape esul ing in a compac s uc u e wi h only 4- old coo dina ed Ti-a oms and wi h one e minal Ti–O bond. In p e ious wo ks,26,27 we pe o med eal- ime calcula ions wi h clus e s as la ge as (TiO2)38 and demons a ed ha , e en being a small clus e , (TiO2)9is he minimal size equi ed o ep oduce adequa ely he elec onic abso p ion spec a o he dye–TiO2sys ems. The (TiO2)9 clus e is a o dable om equency domain TD-DFT calcu- la ions, which allows o a ull analysis o he spec a. These dyes p esen wo di e en con o ma ions espec o he ni ogen a om o he unsa u a ed ings, syn and an i, which ha e been epo ed o be almos isoene ge ic.15 In his wo k, we ha e selec ed he syn con o me , which has also been used in p e ious heo e ical s udies.16,17 Ne e heless, i has been sugges ed om ab ini io calcula ions ha he ab- so p ion spec a o he syn and an i isome s a e almos indis- inguishable o a couma in dye.15 Fo D1, D2, D3, AB1, and AB2, wo di e en con o - ma ional isome s exis . The con igu a ion a he single bond be ween he couma in moie y and he me hine chain can ha e ei he an s-cis o s- ans a angemen . F om p e ious calcu- la ions, we ha e seen ha o NKX-2311, he s-cis isome is sligh ly a o ed, due o he s e ic epulsion be ween he C=O g oup in he couma in and he C-N g oup o he cyanoac ylic acid in he s- ans isome .29 Ne e heless, ene gy di e ences be ween bo h isome s a e insigni ican (less han 0.03 eV). Mo eo e , we ha e checked ha he calcula ed elec onic ab- so p ion spec a o bo h isome s a e almos iden ical. In his wo k, o each dye we ha e chosen he s-cis iso- me whene e possible ( he e is only one possible con o ma- ion o BC1, CE1, BC2, and CE2). The dyes ha e been ad- FIG. 2. Op imized geome ies o he mos p omising dyes adso bed on (TiO2)9: D2 (le ) and CE2 ( igh ). Dyes a e adso bed dissocia ed, in a biden- a e chela ing con igu a ion, wi h he wo dye oxygen a oms bound o he same su ace i anium a om and he dissocia ed hyd ogen bound o an oxy- gen in he cen e o he clus e . so bed dissocia ed, o ming a biden a e chela ing s uc u e, wi h he wo dye oxygen a oms bound o he same su ace i a- nium a om. To keep he sys em elec oneu al, he dissocia ed hyd ogen a oms ha e been bound o an oxygen a om in he cen e o he clus e (Figu e 2). In a p e ious s udy, we ha e demons a ed ha his adso p ion mode is he mos s able one o couma in based dyes.29 Fu he mo e, his chela ing con- igu a ion is also ound o be a o able o dyes con aining ca boxyla e g oups adso bed on ana ase su aces.47 To p edic he new dyes e iciency, we ha e made he use o c i e ia published p e iously.29 On compa ing wo dyes wi h simila s uc u es, he ligh ha es ing would be mo e e icien o he dye wi h a lowe abso p ion h eshold and a wide i s band in he elec onic abso p ion spec um. Fu - he mo e, he elec on injec ion would be mo e e icien o ha dye wi h he highe exci ed s a e ela ed o he semicon- duc o conduc ion band edge. The e o e, we will use h ee c i- e ia o e alua e an imp o emen o he e iciency o he new dyes: he elec onic abso p ion spec a (posi ion and wid h o he i s band and abso p ion h eshold) and he posi ion o he LUMO le el ela ed o he conduc ion band. III. RESULTS A. Elec on-dona ing subs i uen s in d posi ion The simula ed abso p ion spec a om eal- ime TD- DFT calcula ions o adso bed D1, D2, and D3 dyes a e shown in Figu e 3(le ). Adso bed NKX-2311 abso p ion spec um has been included o compa ison. The D1 and D3 spec a show a p o ile e y simila o he NKX-2311 one; hey p esen a main in ense band wi h a small shoulde a highe ene gies and he main peak maximum almos in he same po- si ion. On he o he hand, o D2, he in oduc ion o a –NH2 g oup in d posi ion p oduces a edshi o abou 0.1 eV on he abso p ion maximum posi ion and also a edshi on he abso p ion h eshold compa ed o NKX-2311, ex ending ab- so p ion in o he isible egion. Fo adso bed D1 and D3, bo h exci a ions and o bi al schemes a e e y simila o hose o NKX-2311, which was desc ibed in de ail in a p e ious wo k.29 In gene al, o hese h ee adso bed sys ems, molecula o bi als a e ei- he comple ely localized on he clus e o ully localized on he dye molecule. Howe e , o he adso bed D2, mo e 194702-4 Sánchez-de-A mas e al. J. Chem. Phys. 136, 194702 (2012) FIG. 3. Simula ed eal- ime TD-DFT spec a (le ) and HOMO and LUMO* ene gies ( igh ) o adso bed NKX-2311, D1, D2, and D3. In he ene gy scheme, he o bi al ene gies ha e been shi ed o se he ze o ene gy o he lowe edge o he semiconduc o conduc ion band. mixing be ween clus e and dye o bi als is obse ed and e e y i ual o bi al shows some con ibu ion om he clus e . The LUMO+7 o bi al co esponds o he LUMO o he ee dye and i is delocalized o e he whole sys em (Figu e 4), while o NKX-2311, D1, and D3, his o bi al has no con ibu ion om he clus e . F om he e on, o each sys em, we will use he name LUMO* (o L*) o e e o he o bi al o he ad- so bed sys em, which is mo e simila o he LUMO o bi al o he ee dye. The adso bed D2 spec um p esen s a main wide band cen e ed a 2 eV wi h a small shoulde a lowe ene gy (cen- e ed a 1.7 eV). This shoulde co esponds o an exci a ion om he HOMO o bi al, ully localized on he molecule, o LUMO+13 (Figu e 4), an o bi al ully localized on he clus- e (Table II). Howe e , in his band, he e a e mino con ibu- ions om he LUMO+7 (LUMO*) and LUMO+8 o bi als, bo h delocalized o e he whole sys em. The main band in- cludes se e al exci a ions om HOMO o bi al o di e en i - ual o bi als; some o hem delocalized o e he clus e and o he s delocalized o e he whole sys em, such as LUMO*. To comple e he analysis, he HOMO and LUMO* en- e gies o he ou adso bed dyes ha e been ep esen ed in Figu e 3( igh ). In his ep esen a ion, he LUMO o bi al en- e gy has been se o ze o. The LUMO o bi al is, o e e y sys em, an o bi al ully localized on he clus e and e y sim- ila o he same o bi al o he ee clus e . In consequence, he ze o alue co esponds o he lowe edge o he semiconduc- o conduc ion band. Fo all modi ied dyes, he gap be ween HOMO and LUMO is smalle han o he unsubs i u ed dye. The di - H L L+13 L+7 (L*) L L + 7 ( L * ) FIG. 4. Occupied and i ual molecula o bi als o adso bed D2. e ence wi h NKX-2311 HOMO-LUMO ene gy gap is a he small o D3 (less han 0.05 eV) and bigge o D2 (0.25 eV), e lec ing so an impo an edshi o he abso p ion maxi- mum o D2. The dec ease in he HOMO-LUMO ene gy gap is due o a des abiliza ion o he HOMO o bi al, which is max- imum o D2. Mo eo e , o he h ee dyes, LUMO* o bi al is less s able han o NKX-2311. This des abiliza ion is small o D1 and D3 (0.06 eV) bu i is mo e impo an o D2 (abou 0.1 eV). The highe he LUMO* o bi al is loca ed in he semi- conduc o conduc ion band, he mo e e icien he elec on in- jec ion is. Rega ding his c i e ion, we migh only expec a no iceable imp o emen o he e iciency o he D2 dye. Fo his dye, LUMO* is loca ed highe ela ed o he lowe edge o he TiO2conduc ion band, so i is loca ed in a zone whe e he e ec i e densi y o s a es is highe . This makes he elec on injec ion easie and i also explains ha he e is mo e mixing be ween he clus e and dye o bi als o his dye. B. Subs i uen s wi h s e ic e ec We ha e in oduced in o he NKX-2311 molecule he g oups –CH2–CH2–CH2– (1) and –CH2–HC =CH– (2). TABLE II. Assignmen s o elec onic exci a ions o adso bed D2 dye. Only selec ed ansi ions wi h enough oscilla o s eng h a ound he main peak a e included. (H s ands o HOMO and L o LUMO). T ansi ion ene gy Oscilla o (eV) s eng h Wa e unc ion Shoulde 1.67 0.0755 H→L+13 (0.66), H →L+12 (0.15) H→L+7(−0.10) H→L+8(−0.10) Main band 1.82 0.0636 H→L+15 (0.68) 1.94 0.1154 H→L+17 (0.65), H→L+16 (−0.12) 2.06 0.9507 H→L+7 (0.24), H→L+8 (0.24) H→L+17 (−0.23), H→L+6(−0.18) H→L+13 (0.16), H→L+15 (0.14) H→L+19 (−0.14), H→L+21 (0.13) H→L+5 (0.12), H→L+9 (0.11) H→L+16 (−0.11), H→L+20 (0.11) H→L+18 (−0.10) 194702-5 Sánchez-de-A mas e al. J. Chem. Phys. 136, 194702 (2012) FIG. 5. Simula ed eal- ime TD-DFT spec a (le ) and HOMO and LUMO* ene gies ( igh ) o adso bed NKX-2311, AB1, BC1, and CE1. In he ene gy scheme, he o bi al ene gies ha e been shi ed o se he ze o ene gy o he lowe edge o he semiconduc o conduc ion band. These g oups bind o wo di e en C a oms o NKX–2311, leading o he o ma ion o a new ing in he molecule. The in oduc ion o he i s subs i uen (–CH2–CH2– CH2–) does no p oduce impo an changes in he elec onic s uc u e o he elec onic spec um o NKX-2311. The posi- ion o he abso p ion maximum emains almos unal e ed o AB1 and BC1 and is sligh ly blue-shi ed o CE1 (Figu e 5 le ). The main con ibu ion o he i s band and he o - bi al scheme a e also e y simila o hose o NKX-2311. AB1 p esen s a HOMO-LUMO* ene gy gap sligh ly smalle han NKX-2311 and bo h HOMO and LUMO* a e sligh ly mo e uns able han hose o adso bed NKX-2311 ones (Figu e 5 igh ). Howe e , di e ences a e oo small o p e- dic an imp o emen o he cell e iciency. Fo BC1 and CE1, he LUMO des abiliza ion is mo e impo an (0.11–0.13 eV), which would a o elec onic injec ion, bu i en ails an in- c ease o he ene gy gap be ween he HOMO and LUMO* o bi als, which is disad an ageous o he ligh ha es ing p ocess. The e o e, he c i e ia do no pe mi o p edic an imp o emen o he cell e iciency using hese new sensi- ize s. Ne e heless, he in oduc ion o a new ing in he molecule could p e en dye agg ega ion, which would lead o an imp o emen o he cell e iciency. Since ou model con- ains only one dye molecule, we canno s udy agg ega ion e - ec s in his wo k. On he o he hand, he in oduc ion o he second sub- s i uen (–CH2–HC=CH–) p oduces impo an changes in he elec onic abso p ion spec a: he posi ion o he abso p ion maximum edshi s abou 0.2 eV o he h ee new dyes (AB2, BC2, and CE2). Fo he h ee dyes, he spec um p esen s a main band wi h a e y simila shape; i is cen ed a 1.95 eV and has a small shoulde a lowe ene gies (Figu e 6le ). While in oducing a new ing in he molecule, his subs i uen in oduces also a new double bond, which expands he πcon- juga ion in he dye, esponsible o he edshi and he ab- so p ion widening in he isible egion, desi able o sunligh ha es ing. Ne e heless, a highe ene gies he e a e some di e - ences be ween he spec a. In adso bed NKX-2311 spec a, he e is a small shoulde o he main band a 2.8 eV. This shoulde is sligh ly mo e in ense o adso bed AB2 and i be- comes an in ense well- esol ed peak o BC2 and CE2. The assignmen s o he mo e in ense elec onic exci a- ions o adso bed CE2 dye spec um a e shown in Table III. The spec um shows a main band cen e ed a abou 1.9 eV, which p esen s a small shoulde a lowe ene gy cen e ed a 1.6 eV. The main con ibu ion o his shoulde co esponds o an exci a ion om he HOMO o bi al, ully localized on he molecule, o he LUMO+8, an o bi al delocalized o e he whole clus e , bu wi h an app eciable molecule con ibu ion. The main band includes se e al exci a ions om he HOMO FIG. 6. Simula ed eal- ime TD-DFT spec a (le ) and HOMO and LUMO* ene gies ( igh ) o adso bed NKX-2311, AB2, BC2, and CE2. In he ene gy scheme, he o bi al ene gies ha e been shi ed o se he ze o ene gy o he lowe edge o he semiconduc o conduc ion band. 194702-6 Sánchez-de-A mas e al. J. Chem. Phys. 136, 194702 (2012) TABLE III. Assignmen s o elec onic exci a ions o adso bed CE2 dye. Only selec ed ansi ions wi h enough oscilla o s eng h a ound he main peak a e included. (H s ands o HOMO and L o LUMO). T ansi ion ene gy Oscilla o (eV) s eng h Wa e unc ion Shoulde 1.56 0.0336 H→L+8 (0.60), H→L+5 (0.28), H→L+7(−0.21) Main band 1.87 0.4197 H→L+13 (0.40), H→L+5(−0.31) H→L+8 (0.25), H→L+7 (0.20) H→L+12 (0.14), H→L+10 (−0.12) H→L+15 (−0.12), H→L+9 (0.10) H→L+6(−0.10) 1.99 0.1911 H→L+13 (0.55), H→L+15 (0.27) H→L+5 (0.18), H→L+8(−0.14) H→L+7(−0.11) 2.10 0.0748 H→L+15 (0.61), H→L+16 (0.19), H→L+13 (−0.13), H–1→L+4 (0.11), H→L+5(−0.10), H→L+17 (0.10) o bi al o di e en o bi als ully localized in he clus e , wi h- ou any molecula con ibu ion (LUMO+13, LUMO+15). Ne e heless, all exci a ions wi h no iceable in ensi y ha e a ela i ely impo an con ibu ion o he exci a ion om he HOMO o he LUMO+5 o bi al. The la e co esponds o he LUMO o he ee dye (LUMO*), al hough i has a con- side able con ibu ion om he clus e . Rep esen a i e on- ie occupied and i ual molecula o bi als o adso bed CE2 a e shown in Figu e 7. The HOMO-LUMO* ene gy gap o AB2, BC2, and CE2 is smalle han o NKX-2311 (Figu e 6 igh ). The expan- sion o he πconjuga ion in he dye p oduces a des abiliza- ion o he HOMO o bi al. Fo BC2 and CE2, he LUMO* o bi al is also less s able han o NKX-2311, which makes he elec on injec ion p ocess easie o hese wo new dyes. Combining he di e en c i e ia, we p edic ha in oducing a –CH2–HC=CH– g oup in he molecule could lead o an im- p o emen in he cell e iciency, being he mos p omising dye CE2, because i combines he bigge edshi o he spec um maximum posi ion and abso p ion h eshold wi h a mo e im- po an HOMO des abiliza ion and wi h a highe LUMO* o - bi al ela ed o he lowe edge o he semiconduc o conduc- ion band. H L+5 (L*) L+15 L+8 FIG. 7. Occupied and i ual molecula o bi als o adso bed CE2. IV. CONCLUSIONS In summa y, in his wo k, we show how we can employ he elec onic abso p ion spec a (posi ion and wid h o he i s band and abso p ion h eshold) and he posi ion o he LUMO le el ela ed o he conduc ion band as c i e ia o e alua e he e iciency o new couma in based dyes. We ha e in oduced se e al unc ional g oups in he NKX–2311 molecule wi h he aim o imp o ing i s e iciency as DSSC sensi ize . The in oduc ion o a –NH2g oup in d posi ion could lead o an e iciency imp o emen due o an impo an HOMO des abiliza ion and a sligh ly LUMO des abiliza ion, which p oduces a edshi o he abso p ion maximum posi ion and he abso p ion h eshold. The in oduc ion o –CH2–CH2–CH2– does no in oduce signi ican changes in he elec onic s uc u e o NKX-2311, bu i could p e en he o ma ion o agg ega es. Finally, –CH2–HC=CH– p oduces impo an changes bo h in he elec onic spec um and in he elec onic s uc u e o he dye, and we migh expec an imp o emen o he cell e iciency o AB2, BC2, and CE2. In pa icula , he CE2 would be he mos p omising dye, because i combines a edshi in he spec um wi h a bigge HOMO des abiliza ion and a bigge ene gy gap be ween LUMO* and he edge o he semiconduc o conduc ion band. 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